Fax transmissions have become more and more popular, and the volume of information transmitted through fax machines is increasing steadily. This has caused a number of problems, for instance, in that unauthorized transmission of material may occur from a given location, without leaving any trace, or fax machines may be used for criminal activities to bypass police wiretapping of normal telephone lines.
The present invention has as an object to provide a fax monitoring machine which may monitor, record, store and retrieve fax transmissions which intervene between any two fax machines. Thus, it is one object of the present invention to provide a machine which may be located between any two fax machines which are communicating, may receive information transmitted from both fax machines during communication, and may permit to follow a transmission and analyze the material transmitted.
If the apparatus of the invention is used in open analysis of transmission, e.g., in an industrial location, it is not overly important to make sure that the transmitting and receiving fax machines do not detect that the transmission is being followed (however, it is of course necessary to avoid disturbances of the line which may cause a malfunction of the machines). On the other hand, in many other applications, such as for police wiretapping, it is important that the transactions of transmitting and receiving fax machines be monitored and recorded by the monitoring apparatus without making it evident that wiretapping exists. In all cases, it is desirable to avoid any interference with the two communicating machines. It is therefore necessary to provide an interface which is "transparent" to the handshake, and which does not leave any record of its presence, either on the transmitting or the receiving machine, and which cannot be easily discovered by line-analyzing apparatus. Such a monitoring apparatus interface must be activated by the activity taking place on the telephone line between the two monitored machines, must record the transmission intervening between the two machines, but must not interfere with the transmitted signals or transmit any signal which actively takes part in the handshake between the apparatuses. Providing such an interface is one of the purposes of this invention. It should be noted, however, that such a transparent interface will not prevent the operator from voluntarily interfering with the transmission, e.g., to stop the transmission of a non-authorized message, if appropriate interference means are provided.
Thus, it is another object of the invention to provide a fax monitoring apparatus which can effectively monitor transmission of fax messages occurring between two fax apparatuses, without leaving any trace of its presence, either on the transmitting or the receiving apparatus, and without being easily detectable by wiretapping-detecting apparatus.
It is well known in the art to connect two telephone lines, for instance for policing purposes, and to eavesdrop on telephone conversations. However, when monitoring a plurality of lines at the same time, it is necessary to provide means for the automatic identification of activity starting and ending, and for the recording of telephone activity in an automatic manner, since a single operator is not capable of following a large number of telephone lines. In the art there are known devices which can be connected to telephone monitoring and recording apparatus for this purpose, which identify the starting and ending activity of a given telephone line by following the changes in voltage of the line. As it will be understood by a skilled person, using the telephone line, e.g., by lifting the receiver, results in changes of voltage which drops, when the line is used, from its previous value to a lower value. The problem with this type of monitoring is that different telephone lines show different voltages, depending on the type of line, area, position of connection to the telephone line, and so on. Therefore, although nominal voltages of about 48 V should exist on normal telephone lines, in many instances the actual value is much lower, and sometimes even higher. Furthermore, the amount of voltage drop, after the receiver is lifted, is highly dependent on the distance between the receiver and the exchange central. For this reason devices known in the art are provided with adjusting means, normally a potentiometer, for adjusting the reference voltage before initiating monitoring of a telephone line.
These devices known in the art suffer from several considerable disadvantages. First of all, it is necessary to adjust the reference voltage for each monitored telephone line every time the device is allocated to a different monitored line. This is laborious and time-consuming. Furthermore, even when the same telephone line is monitored, fluctuations in the actual voltage of the line exist, so that the actual value may be different from the reference value which has been set by the operator, which results in faulty functioning of the system, missing required activations of the recording apparatus, etc.
It is therefore another object of the present invention to provide a method which overcomes the aforesaid disadvantages, and which can be used for accurately monitoring the activity of a telephone line, without the need for manual settings of reference values.
It is still a further object of the invention to provide an apparatus which can be used for the aforesaid purposes.
It should be noted that by "telephone line" it is intended to include all lines which are adapted to be used for transmitting telephone conversations, including lines which are dedicated to telecopying apparatus or computer transmission or the like. Thus, the outcome of the monitoring of a given telephone line could be a recording of a fax transmission or a voice conversation, or any other type of data transmission, and the actual nature of the transmitted conversation is not important for the purposes of this invention.
The fax monitoring apparatus according to the invention comprises:
One or more line interface means to simultaneously detect and analyze all audio signals produced on any given telephone line by two connected fax machines;
Analysis means to analyze and interpret the said audio signals to form a compressed fax image;
Expansion means to extend the compressed fax image into a bit-mapped image;
Storage means, to store the compressed fax image and/or the bit-mapped image;
Display means to display the bit-mapped image of the original fax transmission; and
Optionally, printing means to provide a hard copy of the transmission or part thereof.
According to one embodiment of the invention, the storage means comprise a temporary memory or buffer, and one or more hard disks. Furthermore, in another embodiment of the invention, the storage means further comprise one or more digital cartridge recorders and, optionally, one or more floppy diskette drive and/or optical disk drive. 0f course, any other mass storage media can be employed, as becomes available from time to time.
According to a preferred embodiment of the invention, the line interface means comprise a non-interfering line interface device comprising a circuit consisting of two symmetrical main branches connected to an isolation amplifier, two or more protectors being connected parallel to the isolation amplifier, the said two protectors being connected to the ground and the total impedance of each branch before the connection to the protector line being substantially smaller than the impedance of the branch after the said connection. Preferably the ratio of the smaller to the greater impedance of each branch is about 1:5.
According to another preferred embodiment of the invention a separate modem is provided for each monitored telephone line. This is different from other existing apparatus in which it is attempted to exploit a single modem for several telephone lines. It has been found that using the same modem for a number of lines is not convenient, because one modem can take care of a single telephone line at a time, and when two different telephone lines are operating at the same time, and fax transmissions are transmitted thereon, one single modem cannot handle both.
The apparatus according to the present invention can further be used to monitor and record mixed voice/fax transmissions. Additionally, the apparatus can be exploited for monitoring and recording telephone transactions which are entirely voice transactions. This, as will be apparent to a person skilled in the art, is very important because in many instances fax transmissions are preceded or followed by voice conversations, or a telephone line which is in principle dedicated to fax transmissions can be used for voice conversations. According to one embodiment of the invention, this is achieved by adding an analogue-to-digital transducer connected in parallel to each modem, to transform audio signal transmitted through the telephone line into digital storable information.
The apparatus can be activated by any suitable method which permits to recognize the activity of the telephone line. One such method which is novel and forms a part of the present invention comprises:
continuously sampling the voltage of the telephone line to be monitored; PA0 calculating the mean value of the voltage from a plurality of samplings taken during a given time interval; PA0 comparing the said mean value of the voltage with a reference voltage defined as the mean voltage obtained during the previous time interval; PA0 replacing the value of the reference voltage with the said mean value; PA0 if the difference between the last calculated mean value and the reference value exceeds a predetermined value:
(a) stopping the replacement of the reference voltage with the mean calculated value of the voltage; PA1 (b) activating signal recording means to record signals transmitted through the telephone line; PA1 (c) continuously comparing the calculated mean value with the reference value, and if the difference between the two compared values is below a predetermined minimal value: PA1 1) deactivating the signal recording means; and PA1 2) resuming the replacement of the value of the reference voltage with the calculated mean value of the voltage, and the subsequent steps.
The signal recording means referred to above can be, in this case, an apparatus according to the invention.
According to a preferred embodiment of the invention, the method further comprises recording and analyzing the telephone number which is dialed by a monitored telephone line, which method includes the steps off
(1) identifying dialing beginning by comparing the calculated mean value of the voltage and the reference voltage, and determining whether the difference is below a predetermined maximal value;
(2) counting the number of pulses of high voltage occurring within about 0.1 second from one another (.+-.a predetermined tolerance), to determine the dialed digit;
(3) identifying the end of one dialed digit by identifying a low-voltage interval of a length exceeding about 0.1 second (.+-.a predetermined tolerance);
(4) recording the said pulses of high voltage or the equivalent digits of the dialed number in the signal recording means.
As in many instances mixed dialing, including pulse and tone signals, is employed, e.g., to activate a telephonic secretary, a preferred embodiment of the invention further comprises providing tone-detecting means to identify and record tone-modulated codes and/or digits.
While the maximal difference between the calculated mean value of the voltage and the reference value can be changed, on the basis of various considerations which are within the scope of the person skilled in the art, it is preferred to initiate recording of telephone line activity when the calculated mean value is equal to or less than 80-85% of the reference voltage.
The mean calculated voltage is obtained, as said above, by averaging a large number of samplings taken during a very short period of time. In a preferred embodiment of the invention, the mean calculated voltage is the arithmetic mean of 200 to 5,000 samplings, preferably about 1,000 samplings taken during a time interval of about 1 second.
The activity of a telephone line may be recorded on a variety of recording media. In a preferred embodiment of the invention the recording means comprise digital recording apparatus, and the activity of the telephone line is saved in the form of a digital record.
The invention further encompasses a device for monitoring and analyzing the activity of a telephone line, which device comprises:
sampling means for continuously sampling the voltage of the telephone line;
calculating and comparing means for calculating and comparing mean values of the line voltage. These calculating and comparing means may be, e.g., a microcomputer or a microprocessor;
activating means for activating signal recording means. These activating means may, again, comprise a microprocessor or a microcomputer; and
signal recording means to record the activity of the telephone line.
Encompassed by the present invention is also a method of monitoring fax communication, which method comprises the steps of:
a) providing line interface means connected to a telephone line to be monitored;
b) providing activating means to detect line activity comprising one of start/stop activity, ringing and dialing pulse and DTMF signalling;
c) providing interpreting means activated by the said activating means;
d) interpreting fax transmissions by means of the interpreting means when line activity is detected;
e) assembling the interpreted fax transmission into compressed fax images;
f) storing the compressed fax images in memory means;
g) retrieving the stored compressed fax images and expanding them to bit-mapped images of the original message; and
h) displaying the bit-mapped images on display means, or printing them out on printing means.
In a preferred embodiment of the invention, the display means comprise a graphic screen, whereby information can be displayed on the screen. The screen can also be exploited to look for information to be printed on the printer.